ntthal: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On Debian possible default full paths are:

/usr/bin/ntthal - from package: primer3

You may install the required package with command apt-get install {package}

Error in other languages:
ntthal: bevel nie gevind nie
ntthal: comandă negăsită
ntthal: comando não encontrado
ntthal: commande introuvable
ntthal: command not found
ntthal: käsku ei ole
ntthal: không tìm thấy lệnh
ntthal: komanda nerasta
ntthal: Kommando nicht gefunden.
ntthal: kommandot finns inte
ntthal: komut yok
ntthal: nie znaleziono polecenia
ntthal: níor aimsíodh an t-ordú
ntthal: no se encontró la orden
ntthal: no s'ha trobat l'ordre
ntthal: opdracht niet gevonden
ntthal: parancs nem található
ntthal: perintah tidak ditemukan
ntthal: príkaz nenájdený
ntthal: příkaz nenalezen
ntthal: команда не найдена
ntthal: командата не е открита
ntthal: 命令找不到
ntthal: コマンドが見つかりません